1. Field of the Invention
The present invention relates to a digital protective relay which responds to amplitude values of electric quantity such as AC voltages, AC current or the like in an electric power system, and more particularly to improvement of frequency characteristics of such relay.
2. Description of the Prior Art
FIG. 1 is a diagram illustrating the principle of a digital processor of AC electric quantity in the prior art disclosed, for example, in Japanese patent application laid-open No. 58-51315, where sampling is performed every 90.degree. electric angle of AC current, and eight data are used and their amplitude values are estimated. In FIG. 1, numerals 1-8 designate sampling values of AC current, numerals 9-16 designate square arithmetic means, numeral 17 designates an adding arithmetic means for estimating the total of results of the square arithmetic means 9-16, numeral 18 designates a dividing arithmetic means for performing division by four, and numeral 19 designates a root arithmetic means for calculating the root of the output value of the dividing arithmetic means 18 and output F.sub.n of the root arithmetic means 19 becomes the amplitude value of current to be estimated.
Next, operation of the digital processor will be described. For convenience of the description, it is assumed that the AC electric quantity is AC current, the maximum value is I, the instantaneous value is i, the fundamental frequency is f.sub.o, and the sampling period is T. Also in order to provide distinct data per sampling time, nT (n=0, 1, 2, . . . , and n=0 is made a prescribed time) is made suffix, and data are expressed as i(0), i(T), i(2T), . . . i(mT). Consequently, when eight data are used, formula (1) applies. ##EQU1##
The sampling period T is fixed to intervals corresponding to 90.degree. with respect to the fundamental frequency f.sub.o of the AC current, and becomes as formula (2) if the frequency is f. ##EQU2##
For example, if the frequency of the AC current is f=f.sub.o =50 Hz, the sampling period becomes T=90.degree..
In general, since the power system is operated at the rated frequency f.sub.o, formula (1) becomes F.sub.n =I and amplitude calculation of current becomes possible and is utilized as an AC overcurrent protective relay for example. For the protective relay to detect fault of the power system, however, since the frequency at the fault generating state is frequently varied from f.sub.o, the amplitude value must be estimated accurately even if the frequency is slightly shifted. Usually for the variation of frequency of about .+-.5%, the error of the amplitude value must be made as small as possible.
If the frequency becomes f=52.5 Hz (increase of 50 Hz by 5%), the period becomes T=94.5.degree. and substituting this to formula (1) it follows that EQU F.sub.n =I{1+0.0737 cos (2.theta.-661.5.degree.)}.sup.1/2 ( 3)
Consequently, a definite value is superposed by oscillation waveform of double frequency. Since the term cos (2.theta.-661.5.degree.) can be varied to +1.0.about.-1.0, it follows that EQU F.sub.n =0.962I.about.1.036I (4)
Consequently, the error of -3.8%.about.+3.6% is generated in comparison to the amplitude value calculation at the rated frequency 50 Hz.
Since the digital operation processor is constituted as above described, the amplitude value calculation error is relatively large when the frequency is varied about .+-.5%. Also since the eight sampling values are used, the time until outputting of the results must be that corresponding to 90.degree..times.8=720.degree.. Further, since data of large amount are used, the memory amount required for the calculation processing becomes large.